The invention is directed to a single mode optical waveguide fiber having controlled negative total dispersion and a relatively large effective area. In particular, the single mode waveguide has a total dispersion which is less than -100 ps/nm-km.
Several factors have combined to make the wavelength range, 1500 nm to 1600 nm, most preferred for telecommunication systems incorporating optical waveguide fiber. These are:
the availability of reliable lasers in the wavelength window around 1550 nm; PA1 the invention of the optical fiber amplifier having an optimum gain curve in the wavelength range 1530 nm to 1570 nm; PA1 the availability of systems capable of wavelength division multiplexing of signals in this wavelength range; and, PA1 the availability of waveguide fibers having a low dispersion to compliment the very low attenuation over this wavelength range. PA1 having a length which is a small fraction, e.g., less than 15%, of the link length; PA1 which is sufficiently low in attenuation to eliminate the need for additional signal amplification solely to offset the compensating waveguide fiber attenuation; and, PA1 which has an effective area sufficiently large to preclude non-linear dispersive effects in the compensating waveguide fiber from being a limiting factor. PA1 G.sub.nl =n.sub.2 /A.sub.eff (exp[D.sub.1 .times.L.sub.1 /D.sub.d /.alpha.]-1)/.alpha., where n.sub.2 is the non-linear refraction coefficient, D.sub.1 is the dispersion of the portion of the waveguide optimized for operation around 1310 nm, L.sub.1 is the length corresponding to D.sub.1, D.sub.d is the dispersion of the compensating waveguide fiber and .alpha. is the attenuation of the dispersion compensating fiber. This expression for G.sub.nl derives from a base definition G.sub.nl .about.n.sub.2 /A.sub.eff (Effective length.times.Output Power). The effective length and output power are expressed in terms of waveguide fiber length and attenuation, .alpha.. The compensating waveguide fiber is introduced into the equation via the requirement D.sub.1 .times.L.sub.1 =D.sub.d .times.L.sub.d. G.sub.nl is a useful quantity in evaluating the efficiency of a link because it is a combination of system factors such as system architecture, amplifier spacing, D.sub.d /.alpha., and, n.sub.2 /A.sub.eff.
These advances in technology make possible very high information rate, multi-channel telecommunication systems which have a large spacing between stations where the signal is electronically regenerated.
However, many telecommunication systems installations pre-date the technological advances which make 1550 nm the most preferred operating window. These earlier systems were designed primarily for use over a wavelength range centered near 1310 nm. The design includes lasers which operate at wavelengths near 1310 nm and optical waveguides which have a zero dispersion wavelength near 1310 nm. The waveguide fiber, in these systems, does have a local attenuation minimum near 1310 nm, but the theoretical minimum at 1550 nm is about half that at 1310 nm.
A strategy has been developed to make these older systems compatible with the new laser, amplifier, and multiplex technology. As disclosed in U.S. Pat. No. 5,361,319, Antos et al., ('319, Antos) and discussed further in the references noted therein, an essential feature of this strategy is to overcome the relatively high total dispersion by inserting into each waveguide fiber link a length of waveguide fiber which compensates for the total dispersion of the link at 1550 nm. The term "link" used herein is defined as the length of waveguide fiber which spans the distance between a signal source, i.e., a transmitter or an electronic signal regenerator, and a receiver or another electronic signal regenerator.
The '319, Antos patent recites a dispersion compensating waveguide fiber having a core refractive index profile which provides a dispersion at 1550 nm of about -20 ps/nm-km. The dispersion sign convention common in the art is that a waveguide dispersion is said to be positive if shorter wavelength light has a higher speed in the waveguide. Because the dispersion at about 1550 nm of a waveguide fiber, having a zero dispersion wavelength near 1310 nm, is about 15 ps/nm-km, the length of dispersion compensating waveguide fiber required to fully compensate for total dispersion at 1550 nm is 0.75 of the original link length. Thus, for example, a 50 km link of waveguide fiber has a total dispersion at 1550 nm of 15 ps/nm-km.times.50 km=750 ps/nm. To effectively cancel this dispersion, a length of dispersion compensating waveguide fiber of 750 ps/nm.div.20 ps/nm-km=32.5 km is required.
The additional attenuation introduced into the link by the dispersion compensating waveguide would have to be offset by means of an optical amplifier. The introduction of additional electronic regenerators into the link would not be cost effective. Further, the cost of the dispersion compensating waveguide fiber is a significant fraction of the total waveguide fiber cost. The long lengths of dispersion compensating waveguide required must be formed into an environmentally stable package which can take up considerable space.
Because the compensating waveguide fiber design usually has more refractive index modifying dopant in the core region, the attenuation is, in general, higher relative to the standard waveguide fiber in a link.
The higher signal power level, made possible by improved lasers and by optical amplifiers, as well as wavelength division multiplexing, increases the possibility that link length or data transmission rate may be limited by non-linear optical effects. The impact of these non-linear effects can be limited by increasing the effective area (Ae.sub.ff) of the fiber. The effective area is
A.sub.eff = 2.pi.(.intg.E.sup.2 r dr).sup.2 /(.intg.E.sup.4 r dr), where the integration limits are 0 to .infin., and E is the electric field associated with the propagated light. The distortion due to non-linear effects depends upon a term of the form, P.times.n.sub.2 /A.sub.eff, where P is the signal power, and, n.sub.2 is the non-linear index constant. Thus, in the design of a dispersion compensating waveguide fiber, care must be taken to insure that A.sub.eff of the compensating fiber is large enough so that the compensation fiber does not cause significant non-linear effects in the link. If A.sub.eff of the compensating fiber is smaller than that of the original fiber in the link, the compensating fiber may be placed at a link location where signal power is lower and thus non-linear effects minimum. Also, in many links the smaller A.sub.eff compensating fiber is a small fraction of the overall link length and so does not contribute significantly to non-linear distortion of the signal.
Thus, there is a need for a dispersion compensating optical waveguide fiber:
Definition
The effective area is A.sub.eff = 2.pi.(.intg.E.sup.2 r dr).sup.2 /(.intg.E.sup.4 r dr), where the integration limits are 0 to .infin., and E is the electric field associated with the propagated light. PA0 The non-linear discriminator factor is defined by the equation